1. Field of Invention
The present invention relates to photomask and the method of manufacturing photomask. More particularly, the present invention relates to a photomask and the method of manufacturing the photomask with an alignment mark region layout.
2. Description of Related Art
In the manufacture of integrated circuit, photolithography process is used to transfer patterns from a photo mask having customized circuit patterns to thin films formed on a wafer. The image transfer process comprises steps of forming a photoresist layer on a non-process layer, illuminating the photoresist layer through a photo mask having the customized circuit patterns, developing the photoresist layer and then etching the non-process layer by using the patterned photoresist layer as a mask. Hence, the image transfer process is accomplished. For a well-manufactured integrated circuit product, the image transfer process mentioned above is performed several times to transfer the circuit patterns to each non-process layers to form the electrical circuit device. Therefore, it is important to align the successive patterned layers to reduce the misalignment errors as the critical dimension of the semiconductor device becomes smaller and smaller.
To achieve proper alignment, at least two alignment marks are formed at the area around the wafer peripheral region. Typically, the alignment marks are located on opposite sides of the wafer diameter and the alignment marks include a series of parallel trenches covering a rectangular or square area. During each subsequent level of manufacturing a multi-level semiconductor device, it is undesired to overwrite the alignment mark so that a portion of the photoresist layer over the alignment marks is covered by mask blades during exposure process. However, the shadow effect happens to transform a portion of the photoresist layer covered by the mask blades near the edge of the mask blades into a shadow region. That is, the portion of the photoresist layer covered by the mask blades near the edge of the mask blades is underexposed due to diffraction. Therefore, after a develop process, the partial underexposed portion of the photoresist layer is removed by the developer and leaves the photoresist residual around the alignment marks. Hence, the photoresist residual will produce undesired residual and defects after photo or etching process.
Accordingly, in the conventional photolithography process, after the stepper performs an exposure process on a shot region next to the alignment mark, it is necessary to perform several exposure processes to further clean out the shadow region around the alignment mark in order to remove the possible photoresist residual in the later develop process. Therefore, the time for transferring the pattern from the photomask to the layer formed over the wafer is increased. Hence, the throughput is decreased.